One important area in the technical field of mobile radio communications relates to the handover (HO) of User Equipment (UE) from a (serving) source cell to a (neighboring) target cell and the optimization of parameters controlling the HO. In this technical field Self Optimizing Networks (SON) and in particular Mobility Robustness Optimization (MRO) are objects of high interest.
The object of MRO is to optimize network configuration parameters (e.g. handover (HO) trigger parameters, HO thresholds or timers) such that the number of Radio Link Failures (RLFs) due to problematic HOs is reduced. A HO is triggered by measurement events reported by a UE, wherein the measurement events in turn are triggered by neighbor and serving cell measurements. In case of a simple relative comparison of a signal strength from a serving Base Station (BS) with a signal strength from a neighboring potential target BS only a single offset parameter has to be specified for the HO trigger. In case of two different Radio Access Technologies (RATs) (and sometimes also for two different frequencies in the same RAT) absolute signal values of source cell and target cells have to be considered, i.e. two thresholds have to be fulfilled simultaneously for triggering a HO event.
HO problems which might even cause RLFs may occur in case the thresholds used have an inappropriate value. 3GPP has specified three different categories of HO problems which may cause a RLF:
(a) Failures due to too late HO triggering
(b) Failures due to too early HO triggering
(c) Failures due to HO to a wrong cell
A further HO problem which typically does not cause RLFs but which causes a lot of signaling load are so called ping-pong handovers which occur in particular if a UE is located within a border region between a first BS and a second BS.
There may be a need for improving the handover behavior of a mobile radio communication network.